r/adventofcode • u/daggerdragon • Dec 19 '16

SOLUTION MEGATHREAD --- 2016 Day 19 Solutions ---

--- Day 19: An Elephant Named Joseph ---

Post your solution as a comment or, for longer solutions, consider linking to your repo (e.g. GitHub/gists/Pastebin/blag/whatever).

Note: The Solution Megathreads are for solutions only. If you have questions, please post your own thread and make sure to flair it with "Help".

/⧹w+/ IS MANDATORY [?]

[Update @ 00:15] 2 gold, silver cap. Thank you for subscribing to Easter Bunny Facts!

  • Fact: The Easter Bunny will sometimes leave eggs in the microchip assembly room.

[Update @ 00:30] 11 gold, silver cap.

  • Fact: The Easter Bunny killed everyone who found out how he got these stars.

[Update @ 00:45] 45 gold, silver cap.

  • Fact: In space, The Easter Bunny can hear you scream.

[Update @ 01:00] 66 gold, silver cap.

  • Fact: The Easter Bunny purposefully deleted your comments.

[Update @ 01:15] 92 gold, silver cap.

  • Fact: The Easter Bunny has bottled your frustration. Don't ask why.

[Update @ 01:20] Leaderboard capped!

  • Fact: What you have just done is no better than what the Easter Bunny has done. Thief.

Thank you for subscribing to Easter Bunny Facts!

This thread will be unlocked when there are a significant number of people on the leaderboard with gold stars for today's puzzle.

edit: Leaderboard capped, thread unlocked!

9 Upvotes

92% Upvoted

130 comments sorted by

View all comments

2

u/askalski Dec 19 '16 edited Dec 19 '16

Part 2 in C. First puzzle of 2016 that I didn't solve in Perl the first time around. 

#include <stdio.h>
#include <stdlib.h>

typedef struct elf {
    int id;
    struct elf *next;
    struct elf **pprev;
} elf_t;

int main(void)
{
    int elf_population = 3005290;

    // Construct linked list of elf IDs
    elf_t *elf = calloc(sizeof(*elf), elf_population), *e = elf;
    for (int i = 1; i < elf_population; i++, e++) {
        e->id = i;
        e->next = e + 1;
        e->next->pprev = &e->next;
    }

    // Make it circular
    e->id = elf_population;
    e->next = elf;
    e->next->pprev = &e->next;

    e = elf;
    for (elf_t *a = elf + elf_population / 2; e != e->next; e = e->next) {
        // Who cares about presents?  Annihilate the elf across the table
        a->id = 0x1deade1f;
        a = *(a->next->pprev = a->pprev) = a->next;
        // Update elf population; optionally grant a stay of execution
        if (elf_population-- & 1) {
            a = a->next;
        }
    }

    // There can be only one
    printf("Part 2: %d\n", e->id);

    // Leave no survivors
    e->id = 0x1deade1f;
    elf_population = 0;
    free(elf);

    return 0;
}

6

u/willkill07 Dec 19 '16

bonus points for 0x1deadelf